A four-second heartbeat to a file that was never there

What was wrong before this entry: a telemetry readout measured its own round-trip latency by sending a HEAD request every four seconds. It sent that request to the site's favicon. The site has no favicon — none has been designed — so every probe was a 404, and every 404 was an edge invocation that did real work and returned an error page.

It ran forever, on every open tab, including tabs that were hidden and drawing nothing. The number it produced was real: it was the honest latency of a 404. It was simply not a measurement of anything anyone wanted.

The cure was three changes, each small. The target became a one-byte static file that actually exists. The interval went from four seconds to twenty. And the probe was gated on two conditions at once: the element being visible, and the document not being hidden.

The general defect is worth naming because it is common and it is quiet: a probe pointed at a missing resource still returns a number. Liveness instruments fail open, and a failing-open instrument reports success while measuring the failure path.